Arc discharge lamps, such as metal halide lamps, are very efficient light sources. Because of the difficulty in beginning the arc discharge various forms of starting aids have been employed. For example, in mercury lamps it has been the practice to use a probe that penetrates the arc chamber. When a voltage is supplied to the lamp a low current discharge occurs between the probe and an adjacent electrode. This low current discharge initiates electrode-to-electrode current conduction, thus starting the lamp. This technique has been used also in metal halide lamps, particularly those having fill pressures below 50 torr of a rare gas. The relatively low pressure leads to arc tube blackening during starting and, additionally, the probe must be electrically neutralized after starting to avoid pinch seal failure by electrolysis. Typically, this is accomplished by means of a bi-metal switch.
In recent years the industry has changed to metal halide lamps that use elevated fill pressures that do not use probes. The ballast for this type of lamp produces high voltage starting pulses for ignition. However, these higher-pressure lamps have slow and erratic starting absent some form of starting aid.
One form of starting aid employs radioactive Kr85, which is injected into the arc tube as a low percentage of the total rare gas fill, and which act to initiate breakdown. However, use of radioactive materials requires specialized equipment in manufacturing as well as significant expense to document compliance with regulatory agency control in manufacturing and transportation.
The starting aid usually employed is a sealed glass capsule containing conditions that generate a burst of UV energy. This technique is described in U.S. Pat. No. 4,721,888 to Proud, et al. In practice, electroded UV capsules such as that disclosed in U.S. Pat. No. 4,818,915 and electrodeless capsules such as that disclosed in U.S. Pat. No. 4,812,714 have been used. While these work very well, they are expensive to manufacture and difficult to automate. Another UV source is disclosed in U.S. Pat. No. 5,323,091 in which the UV source is incorporated into the seal area of the arc tube.